160 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



posterior cardinal vein. The fifth and sixth dorsal segments! arteries are shown 

 in plate 6, figure 2. The seventh and eighth are similar to the fifth and sixth; 

 in addition the eighth presents a bulbous swelling which lies against the lateral 

 wall of the medullary tube. The ninth is almost typical, but its ventral branch 

 is cither lacking or indistinguishable. The tenth to seventeenth show bulbous swell- 

 ings (similar to that of the eighth) of various size extending longitudinally along 

 the neural tube. At some places these swellings extend toward one another and 

 probably form the longitudinal anastomoses along the neural tube, which Felix 10 

 has indicated are present in the Robert Meyer embryo No. 300. Evans 7 has shown 

 that such anastomoses exist in the form of a distinct plexus in the chick embryos. 

 In my specimen anastomoses very probably are present, for what appear to be net- 

 works of endothelial cells connecting adjacent segmental arteries are distinguishable 

 in many places. 



The eighteenth to twenty-fourth dorsal segmental arteries seem to be less 

 well developed. They can only be followed with difficulty, owing to their small- 

 ness and to the plane in which the sections are cut. Their origins from the aorta, 

 however, are very apparent. 



Ventral Segmental Arteries. The ventral segmental arteries are paired vessels, 

 but they are found only in the lower segments of the body and their segmental 

 arrangement is not so definite as that of the dorsal branches. According to 

 Evans 8 there is originally a ventral artery for each segment, but those of the 

 upper body-segments degenerate very early. The upper ones together constitute 

 a row of vitelline arteries, which later, by fusion of the individuals of certain pairs, 

 give rise to the unpaired median vessels of the adult. 



In my specimen there are 19 to 20 pairs of ventral segmental arteries in all, 

 including those which go into -the formation of the umbilical arteries and which 

 should undoubtedly be classified as ventral segmentals. They extend from the 

 seventh segment to the tail. The largest ones are placed opposite the seventeenth 

 and eighteenth somites. All of the ventral segmental arteries pass ventrally along 

 the wall of the digestive tract. Anastomoses, such as Felix has shown, can be 

 made out in certain places. Most of the branches above the nineteenth segment 

 can be traced to the yolk stalk and sac, where they become larger and enter into 

 an extensive plexus. Those below the nineteenth segment go into the formation 

 of the umbilical arteries, as described below. 



Lateral Segmental Arteries. Lateral segmental arteries are found opposite the 

 twelfth to nineteenth segments. They are small vessels, directed laterally at 

 right angles to the longitudinal exis of the aorta. They can be traced as far out 

 as the nephric system. In several places connections can be observed between 

 them and the termination of the medial tributary of the posterior cardinal vein 

 (the beginning median subcardinal veins as described by Evans). I have been 

 unable to determine whether or not lateral segmental vessels exist below the nine- 

 teenth segment, owing to the plane of section and to their indefiniteness. 



Tf-nninal Branches. The dorsal aortse terminate in the tail of the embryo 

 by breaking up into distinct networks of small vessels. These plexuses probably 



